The present invention relates to a toner for developing latent electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like, and more particularly to a toner for use in dry type development in the above technical field.
Generally in a dry-type development method, a developer which is prepared by mixing a fine powder-like toner and a carrier is employed for developing latent electrostatic images. The toner comprises a pigment dispersed in a natural or synthetic resin, and a dye which works as an agent for controlling the electric charge of the toner (hereinafter referred to as a charge controlling agent). The carrier is made of glass beads or iron powder. As the development techniques which belong to the category of dry-type development, there are known, for example, cascade development, fur-brush development, magnetic-brush development, impression development and powder-cloud development.
A conventional toner for use in the dry-type development method is prepared by adding a pigment such as carbon black to a natural or synthetic thermoplastic resin, together with a charge controlling agent, and fusing the mixture, cooling the fused mixture and then finely grinding the mixture to finely-divided particles with a particle size ranging from 5 .mu.m to 20 .mu.m.
As charge controlling agents for use in a toner of a dry-type developer, the following charge controlling agents are proposed in Japanese Patent Publication No. 41-2427.
(1) Positive charge controlling agents PA1 (2) Negative charge controlling agents
Fettschwarz HBN (C.I. No. 26150), PA2 Nigrosin (C.I. No. 50415), PA2 Sudantiefschwarz BB (C.I. No. 26150), PA2 Brilliantspiritschwarz TN (made by Farbenfabriken Bayer Co., Ltd.) PA2 Zaponschwarz X (made by Farbwerke Hoechst Co., Ltd.) PA2 Ceresschwarz (R)G (made by Farbenfabriken Bayer, Co., Ltd.) PA2 Chromogenschwarz ETCO (C.I. No. 14645) PA2 Azo Oil Black R (made by National Aniline Div. Co., Ltd.)
The above charge control agents are selected from dyes and are complicated in chemical structure, unstable and susceptible to mechanical frictions and shocks, changes in temperature and humidity, electrical shocks and illumination, by which they are easily decomposed and the charge controlling properties thereof are impaired.
Furthermore, many of the conventional charge control agents are extremely difficult to disperse or dissolve uniformly in thermoplastic resins, so that the quantity of electric charges in each toner particle containing such a conventional charge controlling agent becomes different. Therefore, the charge distribution in the toner particles differs from portion to portion of the toner. As a matter of course, such a toner is not capable of developing latent electrostatic images to visible images precisely corresponding to the latent electrostatic images, and if it is used in practice in a development apparatus, the development performance reliability thereof cannot be guaranteed.